1. Field of the Invention
The present invention relates to a pattern forming method for forming a resist pattern on a substrate having a plurality of divided exposure regions, a method of manufacturing a thin film transistor substrate, a method of manufacturing a liquid crystal display and an exposure mask.
2. Description of the Related Art
Active matrix color liquid crystal displays have been spreading as displays of personal computers and wall-mounted television receivers. An active matrix liquid crystal display has two substrates and a liquid crystal sealed between the substrates. Thin film transistors (TFTs) are formed like a matrix on one of the two substrates as switching elements for driving the liquid crystal at each pixel. Presently, technical studies and product developments are being actively made to promote the spread of liquid crystal displays having larger screens.
In order to manufacture active matrix displays at a lower cost, it is important to form TFT substrates with a smaller number of manufacturing steps and a high yield of manufacture. For this reason, photolithographic techniques have prevailed as the mainstream because of their capability of transferring a multiplicity of patterns at a time. One exposure mask (reticle) is normally used at one patterning step.
In the case of a liquid crystal display having a large screen, since a substrate is large-sized, it is difficult to transfer patterns on the entire substrate at a time because of the structure of an exposure apparatus. For this reason, divided exposure in which an entire area to be patterned on a substrate is divided into a plurality of regions to be exposed is carried out. Divided exposure employs a separate exposure mask for each of divided regions to be exposed. A resist film formed on a wiring layer is shielded from light in its regions other than divided exposure regions to be exposed, and each of the divided exposure regions is exposed using a predetermined exposure mask and is thereafter developed to form a resist pattern covering the entire regions.
In divided exposure, the exposure mask and the substrate are aligned with each other in each of divided exposure regions. Therefore, the exposure mask for each of divided exposure regions may be misaligned relative to the substrate, and the width of an overlap between a source electrode and a gate electrode of a TFT can therefore be different in each of the divided exposure regions. In this case, the TFT formed in each of divided exposure regions has a different parasitic capacitance Cgs between the gate electrode and the source electrode, which results in differences ΔV between pixel potentials of the divided exposure regions and hence differences ΔT in light transmittance between the regions. As a result, differences in luminance are produced on the display screen of the liquid crystal display and are visually perceived as display irregularities.
A method of making such display irregularities less visually perceptible is a pattern forming method in which patterns associated with different exposure masks are arranged such that they are mixed with each other in a part where the patterns are stitched (for example, see Patent Document 4). Each of the exposure masks is laid out such that in a region of the stitched part where exposure is performed twice or more, the pattern in each pixel is formed by one cycle of exposure and is shielded from light during other cycles of exposure.
Incidentally, the documents of the related art are as follows:
Patent Document 1: JP-A-62-105146
Patent Document 2: JP-A-2-143513
Patent Document 3: JP-A-6-324474
Patent Document 4: JP-A-9-236930
Patent Document 5: JP-A-9-298155
Patent Document 6: JP-A-11-174402
Patent Document 7: International Publication Brochure No. WO95/16276
According to the pattern forming method in the related art, in order to reduce differences in the overlap width of the source electrode and the gate electrode of a TFT between divided exposure regions, a relative misalignment (stitching error) of exposures must be made small in each of a source electrode forming layer and a gate electrode forming layer. For this purpose, parameters (X, Y and θ or the like) for positioning of an X-Y stage of an exposure apparatus are corrected based on a value read from a vernier transferred on to a glass substrate using a measuring reticle for adjusting the exposure apparatus.
The accuracy of alignment of an exposure mask used at a photolithographic step with a substrate can vary within accuracy specifications for the same. In aligning methods according to the related art, alignment between an exposure mask and a substrate is corrected on the basis of an average value of alignment errors of a plurality of exposure masks. Therefore, correction has not been necessarily effective for an exposure mask having an alignment error that is far from the average value. The condition of an exposure apparatus may not be constant, and a positioning error of the X-Y stage can change gradually. The positioning error of the X-Y stage can also undergo irregular changes because of changes in the environment such as the atmospheric pressure. Under such circumstances, the pattern forming method according to the related art results in a problem in that parameters for positioning the X-Y stage of the exposure apparatus cannot be sufficiently corrected.